Put Armature Rest Position to T pose
You can change the rest pose in Pose Mode Editor and using the menu Pose > Apply [Ctrl+A] > Apply Pose as Rest Pose.

HOWEVER, you will destroy your animation: this "rest" pose is the base for the animation data, all transformations are made using this bone orientation.

EDIT: 

As @batFINGER wrote above (about a similar thread: https://blender.stackexchange.com/a/53981/15543), I got the procedure working (with Blender 2.78a) in the following way:


Pose your model to the new rest pose (without keyframing);
Run the "script 1" (retarget_pose.py) to generate UI buttons (under Properties Editor > Object Data Tab);
Click "New Rest Pose to Rig" (it will duplicate the bones - inside the same armature - with the new rest pose);
Click "BVH action to new restpose rig" (it will correct every keyframe of the animation);
Manually delete the original bones (as I could not make the "script 2" - the one posted in the thread - work);